Luminescent lithium silicate activated with trivalent cerium

ABSTRACT

A trivalent cerium activated silicate of the formula Li1-xNaxY1pCepSiO4, wherein O &lt; OR = x &lt; OR = 0.90 and 0.0001 &lt; OR = p &lt; OR = 0.25 and having the olivine crystal structure useful in flying spot scanners.

Unite States Patent Schuil 1 Dec. 17, 1974 [5 LUMINESCENT LITHIUM SILICATE 3,400,081 9/1968 Brixner 252/3014 F ACTIVATED WITH TRIVALENT CERIUM 3,715,611 2/1973 Hijman et a1 252/301.4 F

[75] Inventor: Roelof Egbert Schuil, Emmasingel, OTHER UBLICATIONS Eindhoven, Netherlands Bondar et al., Akademiia Nauk, SSSR, Izvestiya Ser [73] Assignee: U.S. Philips Corporation, New FIZ VOL 1057-1061, 1969' York Blasse et al., Journal of Chemical Physics, Vol. 48, I No. 1, pp. 217 222, 1968. [22] Filed July 1972 Blasse et al., I, Journal of Inorganic and Nuclear [2]] Appl. No,; 271,833 Chemistry, Vol. 29, pp. 223l-2241, 1967.

Primary Examiner.lack Cooper [30] Foreign Apphcanon Pnonty Dam Attorney, Agent, or FirmFrank R. Trifari; Norman July 24, 1971 Netherlands 7l 10248 N Spain [52] US. Cl. 252/301.4 F 51 1m. (:1 C09k 1/54 [57] ABSTRACT [58] Field of Search 252/3014 F A trivalent cerium ac iv e silicate of he formula Li, ,Na,Y, ,,Ce SiO wherein O x 0.90 and [56] References Ci d 0.0001 p 0.25 and having the olivine crystal UNITED STATES PATENTS structure useful in flying spot scanners.

2,736,711 2/1956 Gooding et a1 252/301.4 F 3 Claims, 2 Drawing Figures PATENTEDUEEI 914 3,855,143

' gMnm) LUMINESCENT LITHIUM SILICATE ACTIVATED WITH TRIVALENT CERIUM The invention relates to a cathode-ray tube provided with a luminescent screen which comprises a luminescent silicate activated by trivalent cerium. Furthermore the invention relates to a method of manufacturing such a luminescent silicate and to the luminescent silicate itself.

Cathode-ray tubes provided with a luminescent screen which emits radiation upon excitation by electrons generatedin the tube are used for many purposes, for example, for displaying television images, for recording oscillograms and for flying spot scanners. Dependent on their use the luminescent materials to be used in the luminescent screens must have different properties, for example, as regards spectral energy distribution, decay time, saturation, etc. A property which is always desired is that the energy conversion of the incident electrons into radiation is effected with a high efficiency.

For some uses, for example, in flying spot scanners a very important factor is the decay time of the radiation of the luminescent material. (The decay time is to be understood to mean in this case and hereinafter the period during which the intensity of the radiation emitted by the luminescent material, after discontinuation of the electron bombardment, decreases to He times the value of the intensity just before the electron bombardment is discontinued).

In flying spot scanners the luminescent screen exclusively serves as a light source. The electron beam with which the luminescent screen is excited moves in accordance with a given pattern, sometimes referred to as raster, across the luminescent screen in such apparatus. The electron beam is then not modulated. As a result a fast moving light spot of constant intensity is pro-' duced on the screen. The light of this moving light spot is projected onto a document to be displayed, for example, a lantern slide, a film or a security paper and is partly passed or reflected thereby. The passed or reflected light is received by a photo-electric cell in which it is converted into an electrical signal. This electrical signal may then be transmitted by known communication means to an apparatus in which an image of the document is formed.

During scanning in the flying spot scanner the requirement must be imposed that the radiation incident on the photo-electric cell is correlated as much as possible exclusively with the optical absorption at the area of the spot of the document which is to be displayed at that moment. This leads to the requirement that the decay time of the luminescent radiation relative to the period of the electron beam being present at a given area must not be long. If, as is common practice, the rate of scanning the luminescent screen is equal to the rate at which a normal television display screen is scanned, this leads to the requirement that the decay time must be shorter than approximately see.

A number of luminescent materials have been found which satisfy this requirement. One of these, materials, which is frequently used, is .the so-called gehlenite (Ca- Al SiO which is activated by trivalent cerium. The maximum emission of this material upon electron excitation is at approximately 410 nm and the energy conversion efficiency is approximately 5 percent.

A drawback of gehlenite is that, although the decay time is sufficiently short, a considerable quantity of radiation is emitted during a relatively long period after the instant when the intensity has reached the value of He times the maximum intensity. This so-called persistence is still clearly noticeable sometimes after 0.1 second and causes a disturbing electrical signal in the photo-electric cell. A number of silicates activated by trivalent cerium are described in Netherlands Pat. application 6,81 1,326 which persist to a very slight extent only. These silicates are defined by the formula Y Ce Si Q in which p has the value of l or 2 and in which 2.10" s 11$ 2.10. Particularly the silicates with p l are suitable for flying spot scanners as a result of their favourable spectral distribution of the emission (maximum at 400-430 nm) and their high conversion efficiency (approximately 6 percent). The silicates proposed in the above-mentioned Netherlands Patent application can generally be manufactured with difficulty because the formation of these silicates through diffusion reactions between solid materials must be effected at a high temperature. The use of socalled melting salts in this method often gives rise to an unwanted increase of the persistence period of the luminescent silicate.

The object of the invention is to provide a luminescent silicate activated by trivalent cerium which upon electron excitation has a short decay time and substantially does not persist and whose manufacture does not cause special difficulties.

According to the invention a cathode-ray tube is provided with a luminescent screen which comprises a luminescent silicate activated by trivalent cerium and is characterized in that the silicate is defined by the formula Li Na Y ,,Ce SiO in which 0 5 x S 0.90 and 0.0001 5 p 5 0.25, and that the silicate has the olivine crystal structure.

According to the invention a cathode-ray tube comprises a cerium-activated silicate of lithium and of yttrium in which up to a maximum of mol percent of lithium may be replaced by sodium. The silicate of lithium and yttrium (LiYSiO may occur in two crystal structures, namely the olivine structure which is stable at low temperatures and the B-calcium silicate structure which is stable at high temperatures. The transition temperature between these two structures is approximately l,080 C. It has been found that a large part of lithium in LiYSiO, having the olivine structure may be replaced by sodium (namely to approximately 90 mol. percent) while the olivine structure is maintained. The-pure NaYSiO however, has a different crystal structure.

Experiments which have led to the invention have shown that LiYSiO, having the olivine structure constitutes known very efficient luminescent material upon activation by cerium. Upon electron excitation brightnesses are achieved with this material which are approximately 150 percent of that of the abovementioned knownn gehlenite. The decay time of the luminescence is very short and is of the same order as that of gehlenite (shorter than it see) It is found that the luminescent silicate according to the invention as well as the silicates known from the said Netherlands Pat. application 6,811,326 do not show substantially any persistence. The spectral distribution of the emitted radiation of cerium-activated lithium yttrium silicate according to the invention consists of a comparatively broad band having a maximum at approximately 405 nm.

When partly replacing lithium by sodium in a luminescent silicate according to the invention the luminescent properties of the silicates slightly change. The maximum of the spectral distribution of the emitted radiation shifts with increasing sodium content to larger wavelengths to approximately 430 nm for the compound Li Na YSiO -Ce. The decay time and the persistence period remain short. The brightness upon excitation by electrons slightly decreases with increasing sodium content and is found to be of the same order for higher values of x as that of the known gehlenite. A great advantage of the sodium-containing luminescent silicates according to the invention is that they have a very constant brightness when used in cathode-ray tubes during the lifetime of these tubes. Likewise as the known gehlenite, LiYSiO according to the invention which does not contain sodium is found to lose approximately to percent of the initial brightness after an operation period of 1,000 hours in a cathode-ray tube. On the other hand a silicate according to the invention in which, for example, 80 mol percent of lithium is replaced by sodium has a decline in brightness of approximately 1-2 percent after l,000 hours. Therefore cathode-ray tubes according to the invention are preferred which comprise a silicate defined by the above-given general formula in which x has a value of between 0.50 and 0,80.

In the luminescent silicates according to the invention a small portion of yttrium may be replaced by one or more of the rare earth metals, for example, gadolinium or lanthanum. The luminescent properties of the silicate substantially do not change if the olivine crystal structure is maintained. Such a replacement does not, however, yield extra advantages.

Experiments have proved that in case of partial replacement of lithium by potassium in cerium-activated lithium yttrium silicate a luminescent material is produced which in addition to the blue emission exhibits an intensive green emission upon electron excitation. However, the green emission of this material which does not have the olivine structure disappears gradually during operation in a cathode-ray tube. The decline in brightness of this material during operation is very large so that this material is not very suitable for practical uses.

It is to be noted that cerium-activated sodium yttrium silicate which, as stated above, does not have the olivine structure has a brightness upon electron excitation which is only approximately 15 percent of that of lithium yttrium silicate according to the invention. Consequently, this material is not very suitable for practical uses. This likewise applies to cerium-activated lithium yttrium silicate having the B-calcium silicate structure whose brightness upon electron excitation is only a fraction of the brightness of cerium-activated lithium yttrium silicate having the olivine structure.

The cerium content which is denoted by p in the general formula for the luminescent silicates according to the invention is to be chosen within the abovementioned range. for values of p of less than 0.0001 or more than 0.25 materials having too little brightness are obtained. The maximum brightnesses are achieved for values of p between the relatively wide limits of 0.002 and 0.10. In this range, which is preferred, the brightness is found to be only little dependent on the chosen value of p.

As a result of the favourable spectral distribution of the emitted radiation and of the short decay time and persistence period the silicates according to the invention are particularly suitable for use in flying spot scanners. In many cases, particularly for use in flying spot scanners for generating colour television signals, the emission of the luminescent screen of a cathode-ray tube according to the invention is to be increased in the green and red parts of the spectrum by incorporating a further luminescent material in the screen the emission of which luminescent material lies in the said part of the spectrum. Such a material may be mixed, for example, with the silicate according to the invention, but may alternatively be provided in a separate layer on the screen. Of course, such a material must also have a short decay time, namely shorter than 10 sec. Cerium-activated yttrium aluminate having the garnet structure known for Netherlands Pat. application 6,706,095 may be advantageously used for this second material.

A further advantageous use of the luminescent silicates according to the invention is found in oscilloscopes for recording very fast phenomena.

A great advantage of the silicates according to the invention is that they can be readily manufactured at comparatively low temperatures. For this manufacture a method is preferably used in which a mixture is made of oxides of the metals stated in the general formula or of compounds which can produce these oxides together with a quantity of silicon dioxide. This mixture is then heated in air for 0.5 to 5 hours at a temperature of between 600 and l,200 C. After cooling and homogenising of the product thus obtained it is subjected to a second heat treatment in a reducing atmosphere at a temperature below the transition temperature between the olivine crystal structure and the B-calcium silicate crystal structure. This transition temperature is approximately l,080 C.

Due to the presence of lithium oxide and/or a lithium salt in a molten condition in the firing mixture one is not exclusively dependent on a diffusion of solid materials for the manufacture of the silicates according to the invention. Consequently the formation reaction proceeds faster and better than is the case for manufacturing the said known silicates. It has been found that it is generally advantageous to start from a firing mixture which comprises the composite components in quantities corresponding to the stoichiometry of the desired compound. However, lithium is to be used in a small excess. lt is preferred to use yttrium in the firing mixture in quantities which are not smaller than is stoichiometrically required, because a deficiency of yttrium promotes the formation of the B-calcium silicate structure. The firing temperature at the second heat treatment is preferably chosen to be as high as possible, because then luminescent silicates having the maximum brightnesses are obtained as will be proved hereinafter. At this second heat treatment the temperature must, however, not exceed the said transition temperature.

If necessary, the luminescent silicate obtained by the method described above may be ground until a desired mean grain size is obtained. The luminescent silicates according to the invention are found to have the advantage that such a grinding operation has only a very slight influence on the brightness of these materials. This is in contrast with many known luminescent materials which exhibit a strong decrease in brightness as a result of a grinding operation.

The invention will now be further described with reference to some examples, a number of measurements and a drawing.

FIG. 1 of the drawing diagrammatically shows a cathode-ray tube according to the invention.

FIG. 2 of the drawing shows in a graph the spectral energy distribution of the emitted radiation of a luminescent silicate according to the invention upon electron excitation.

EXAMPLES:

I. A mixture is made of 2.458 g SiO (contains 2.34 percent by weight of 4-628 2 rss om a 0.084 g LiCl 1.476 g Li CO This mixture is heated in air for 2 hours in a furnace at a temperature of 1,l00 C. After cooling the product obtained is ground and subjected to a second heat treatment for 2 hours in a reducing carbon monoxidecontaining atmosphere at a temperature of l,070 C. The reducing atmosphere is obtained by placing a quantity of carbon in the vicinity of the firing mixture in the furnace. After cooling and, if necessary, light grinding and sieving the product is ready for use in a cathode-ray tube according to the invention. The luminescent silicate obtained is defined by the formula LlY0 u9C0 SIO It has been shown with the aid of X ray diffraction analysis that the silicate has the olivine crystal structure. Upon electron excitation in a cathode-ray tube this silicate has abrightness which is approximately 150 percent of that of the known gehlenite. The spectral distribution of the emitted radiation of the silicate has a maximum at 405 nm. The silicate has a decay time of 70 nsec and does not show substantially any persistence. Upon electron bombardment under standard circumstances in a cathode-ray tube which can be disassembled the silicate manufactured in accordance with this example is found to have a decline in brightness of 13 percent. The decline in brightness of the known gehlenite under these circumstances is substantially equally large.

11. Operations are carried out in the same manner as described in example I in which, however, the said quantity of Li CO is replaced by 0.591 g li CO and 1.272 g Na CO A silicate defined by the formula Li Na Y Ce SiQ, having the olivine crystal structure is obtained. The brightness of this silicate is found to be of the same order as that of the known gehlenite. The silicate has a decay time of 80 nsec. and does not show substantially any persistence. The decline in brightness under standard conditions in a cathode-ray tube which can be disassembled is found to be only 2 percent.

111. In the same manner as described in example I a luminescent silicate is obtained which is defined by the formula Li Na y Ce SiO which has the olivine structure. The quantity of I .i CO mentioned in example l is replaced by 0.296 g Li CO and 1.696 g Na C0 The brightness of this silicate is substantially the same as that of the known gehlenite. The silicate has a decay time of 100 nsec and shows substantially no persistence. The maximum of the spectral distribution of the emitted radiation is located at approximately 430 nm. The decline in brightness under standard conditions in a tube which can be disassembled is found to be substantially 0 percent.

IV. To check the influence of the cerium content p a number of silicates is manufactured in a manner completely analogous to example I in which, however, p has the valves of 0.005, 0.02 and 0.04. All these materials are found to have substantially the same brightness, namely approximately percent of the brightness of the known gehlenite. Also the location of the maximum of the spectral distribution is found to be the same for these materials, namely 405 nm.

V. The influence of the firing temperature at the second (reducing) heat treatment on brightness is checked by repeating the method of example I several times in which, however, different values of the firing temperature T at the second heat treatment are used. The table below gives the relative brightness H (in arbitrary units) of the silicate obtained for each value of T. It is clearly shown that the maximum brightnesses are achieved at temperatures near the transition temperature between the olivine and the B-calcium silicate structure. If the firing temperature T esceeds this transition temperature, lower brightnesses are obtained.

In FIG. 1 of the drawing 1 denotes the wall of the cathode-ray tube. 2 denotes the face plate which is coated on its inner side with a luminescent layer 3 which comprises a luminescent silicate according to the invention.

In FIG. 2 of the drawing a graph is shown in which the wavelength A is plotted in nm on the abscissa and the radiated energy E is plotted in arbitrary units on the ordinate. The curve shows the spectral energy distribution of the silicate according to example '1 upon electron excitation.

What is claimed is:

1. A luminescent silicate having the olivine crystal structure, said silicate being activated by trivalent cerium and having the formula Li, ,Na Y, Ce SiO wherein 0.50 S x S 0.80 and 0.0001 5 p S 0.25.

2. The luminescent silicate of claim 1 wherein 0.002 5 p S 0.10.

3. A method of manufacturing a luminescent silicate activated by trivalent cerium, defined by the formula Li ,Na Y, ,,Ce,,SiO wherein x and p have the values recited in claim 1 wherein a mixture of oxides of the metals stated in said formula or of compounds which can produce these oxides, together with a quantity of silicon dioxide are heated in air for 0.5 to 5 hours at a temperature of between 600 andl ,200 C and then the product thus obtained is heated, after homogenizing, in a reducing atmosphere for 1 to 5 hours at a temperature of from about 1,070 C up to but not including the transition temperature between the olvine crystal structure and the ,B-calcium silicate structure. 

1. A LUMINESCENT SILICATE HAVING THE OLIVINE CRYSTAL STRUCTURE SAID SILICATE BEING ACTIVATED TRIVALENT CERIUM AND HAVING THE FORMULA LI1-XNAXY1-PCEPSIO4 WHEREIN 0.50 $ X $ 0.80 AND 0.0001 $ P $ 0.25.
 2. The luminescent silicate of claim 1 wherein 0.002 < or = p < or = 0.10.
 3. A method of manufacturing a luminescent silicate activated by trivalent cerium, defined by the formula Li1 xNaxY1 pCepSiO4 wherein x and p have the values recited in claim 1 wherein a mixture of oxides of the metals stated in said formula or of compounds which can produce these oxides, together with a quantity of silicon dioxide are heated in air for 0.5 to 5 hours at a temperature of between 600* and1,200* C and then the product thus obtained is heated, after homogenizing, in a reducing atmosphere for 1 to 5 hours at a temperature of from about 1, 070* C up to but not including the transition temperature between the olvine crystal structure and the Beta -calcium silicate structure. 